The present invention relates to heat/mass exchange devices and, more particularly, to a blood oxygenator construction which optimizes the ratio of the priming volume to the exchange surface of the oxygenator, which provides even distribution of blood within the exchanging chamber and enhances secondary flows within the main blood flow.
Several concepts and approaches for constructing heat/mass transfer devices are known. One of these is based on the membrane concept. According to this concept, a treating medium, for example oxygen, and a medium to be treated, for example blood, flow across opposite surfaces of a semi-permeable membrane. The membrane keeps the main bulk of the two mediums apart but heat and/or very fine particles are nevertheless transferred across the membrane by conduction, diffusion or similar processes.
An extensive background discussion of the art which is related to the present invention is provided in U.S. Pat. No. 4,111,659 entitled "Mass and Heat Transfer Exchange Apparatus" in the name of Bowley. The contents of that patent are incorporated herein by reference.
In relation to blood oxygenators, special problems are encountered. The blood of a patient is diverted out of the body and into the oxygenator where oxygen is added to it and carbon dioxide is extracted from the blood. The blood which flows in this extra corporeal circulation path must be sufficiently oxygenated and decarbondioxidized to sustain the patient.
As a criterion of efficiency of a blood oxygenator, the ratio K.sub.o =Q.div.V may be used, where Q is the flow rate through the oxygenator with given parameters and conditions such as t.degree. equals temperature, PO.sub.2 equals oxygen tension, P.sub.CO2 is CO.sub.2 tension, %HbO.sub.2 is oxygen saturation, pH of blood, as it enters and leaves the apparatus and V is the priming volume.
This formula means that the lower the priming volume (V) is of an oxygenator that provides a given amount of fully oxygenator blood at given conditions, the higher the value of K.sub.o and consequently, the more efficient it is.
High value of K.sub.o may be achieved by constructing an apparatus in which the following design features are present:
(a) Geometrically built in K'=A.div.V, where A is the surface area of mass/heat exchange and V is the priming volume;
(b) Provisions for enhancement of secondary flows within the main to-be-treated medium flow. In the case with blood, the secondary flows must not create high shear stresses which lead to blood trauma. The secondary flows are to provide constant renewal of the membrane adjacent zero velocity layer of the blood flow; and
(c) Uniform distribution of the to-be-treated medium through all and every passageway of the exchanging chamber, i.e. absence of a path of least hydraulic resistance.
The present invention is directed to a mass and heat exchange device which provides all the above mentioned design features, which make it extremely efficient.
The previously-mentioned patent to Bowley discloses an exchange apparatus which includes a large film with spherical embossment on both sides of the film. Tube-like passages criss-cross the interior of the film through which pressurized oxygen flows. The film is rolled up to form a cylindrical body which is placed in an oxygenating chamber. As pressurized oxygen is forced through the interior passages in the film, the spherical embossments jut out and create interlayer passages through which blood can flow. Oxygen and carbon dioxide flow through the semi-permeable material of which the film is made to achieve oxygenation in the conventional manner.
The apparatus described in the foregoing patent is deficient in that it does not attempt to optimize the priming volume which, in this case, is comprised of the volume between the layers of the film. Moreover, because the film is rolled up in a haphazard manner, the actual contact points between the embossed spherical protrusions of the film cannot be predicted accurately. Therefore, optimum mixing and uniform flow distribution of blood is not achieved.
U.S. Pat. No. 4,351,797 discloses a transfer member assembly which comprises an assembly of tubular conduits through which blood is passed. This device is not designed to optimize and reduce the priming volume. Similarly, U.S. Pat. No. 3,769,163 to Bloomfield discloses a blood oxygenator in which a plurality of tubes are employed. Bloomfield's device is unlike the present invention in that it does not teach the construction and advantage to be derived from the specially shaped and arranged tubes of the present invention.
Other patents which are related to the subject matter of the present invention, but more remotely than the foregoing patents, include U.S. Pat. Nos. 3,777,384, 3,893,926; and 3,768,977.